1. Field of the Invention
This invention relates to an exhaust control system for an engine with turbochargers, and more particularly to an exhaust control system for an engine which is provided with a plurality of turbochargers and in which a part of the turbochargers are operated solely in the operating range of the engine where the intake air mass flow is relatively large.
2. Description of the Prior Art
As disclosed, for instance, in Japanese Unexamined Utility Model No. 60(1985)-178329, there has been known an engine which is provided with a pair of turbochargers, a primary turbocharger and a secondary turbocharger. In the engine, the turbines of the turbochargers are disposed in parallel in an exhaust passage and the blowers of the turbochargers are connected to an intake passage of the engine. An exhaust cutoff valve is provided in the exhaust passage upstream of the turbine of the secondary turbocharger and is closed in a small intake air mass range where the intake air mass flow is smaller than a preset value, thereby making inoperative the secondary turbocharger and concentrating exhaust gas from the exhaust passage to the turbine of the primary turbocharger in order to ensure a high supercharging pressure. On the other hand, in a large intake air mass range where the intake air mass flow is larger than a preset value, the exhaust cutoff valve is opened so that exhaust gas from the exhaust passage is fed to the turbines of both the turbochargers thereby causing the secondary turbocharger to operate together with the primary turbocharger and obtaining a proper supercharging pressure.
In order to optimize the supercharging pressure characteristic curve in such an engine provided with a pair of turbochargers, a waste gate valve is provided in a waste gate passage which is provided in the exhaust passage and bypasses the turbine of the primary turbocharger and the waste gate valve is opened to relieve a part of the exhaust gas fed to the primary turbocharger when the pressure of intake air in the intake passage downstream of the blower of the primary turbocharger becomes not lower than a predetermined value.
In such a turbocharged engine with a waste gate valve, there has been a problem that at a time the operating condition of the engine shifts from the small intake air mass range to the large intake air mass range (e.g., during acceleration), it takes a long time for the rotational speed of the secondary turbocharger to increase to a level at which the supercharging effect can be obtained since the secondary turbocharger has been stopped or has been rotating at a very low speed until that time, and accordingly the vehicle cannot be satisfactorily accelerated.